1. Fields of the Invention
The present invention relates to a liquid crystal display device and a method of driving the same, and a liquid crystal projector apparatus, and more particularly, to a liquid crystal display device and a method of driving the same, and a liquid crystal projector apparatus, wherein video signals of a sub-frame are made into video signals having a predetermined polarity with respect to an electric potential of a counter electrode of a pixel matrix.
2. Description of the Related Art
Liquid crystal display devices are one type of electronic display device. Liquid crystal display devices having an active matrix type liquid crystal display device and a high performance display quality are generally used as monitors for PCs and liquid crystal display devices for a projector. In the active matrix type liquid crystal display device, TFTs (Thin Film Transistors) as active devices are provided in pixels, respectively, (hereinafter referred to as pixel TFTs) to thereby construct a liquid crystal panel.
A liquid crystal panel using polysilicon TFTs as TFTs of the active matrix type liquid crystal display device has a superior advantage in that a part of a peripheral circuit can be formed on a glass substrate concurrently with the pixel TFTs.
Because of this superior advantage, many liquid crystal panels using the polysilicon TFTs are used in liquid crystal display devices for which miniaturization and high definition are required.
In particular, in a liquid crystal display device for a projector for which high definition equal to or more than 1,024×768 pixels is required in a liquid crystal display device having a diagonal size equal to or smaller than 1 inch (2.54 cm), the only type of liquid crystal display devices utilized are those having a liquid crystal panel using polysilicon TFTs.
High picture quality is required for a liquid crystal display device for a projector in order to enlarge and project small images on a screen having a diagonal size of about 100 inches. This degree of picture quality is equal to or higher than that of a liquid crystal display device for a PC. In order to obtain this high picture quality, it is necessary to increase luminance and contrast.
Generally for driving a liquid crystal device, A.C. driving is used in which the polarity of a voltage applied to a pixel is changed every frame. In accordance with this A.C. driving, it is possible to avoid the disadvantage which occur when a D.C. voltage is applied to liquid crystal molecules.
Generally, the A.C. driving used in the liquid crystal display device for a projector is a gate line inversion driving. This gate line inversion driving is a driving method in which the polarity of a voltage applied to a gate line is alternately changed on every other row of a liquid crystal pixel matrix, and moreover, the polarity thereof is inverted in frames.
In accordance with this driving method, there is provided a superior advantage in that the flicker can be reduced, and moreover, the longitudinal crosstalk due to the leakage currents in pixel TFTs can also be reduced.
However, if a liquid crystal display device is operated by utilizing the gate line inversion driving method, then the video signals applied to pixels belonging to a particular gate line precedingly driven within a pixel matrix are different in polarity from those video signals applied to pixels belonging to a gate line which is subsequently driven. Hence, a large transverse electric field is generated between the pixel electrodes. The transverse electric field in this case means the electric field generated in a direction with which the pixel electrodes extend along a glass substrate or a liquid crystal layer.
The transverse electric field disturbs the orientation of liquid crystal molecules in a pixel boundary portion, thereby causing light leakage. If light leakage is caused, then the contrast is remarkably reduced and the picture quality is degraded.
As means for avoiding generation of the above-mentioned transverse electric field, heretofore, a metal or the like which does not transmit light is arranged in a portion of generation of the above-mentioned light leakage in order to block the leakage light, thereby preventing a reduction in contrast.
The provision of the above-mentioned metal or the like reduces the pixel area and reduces an aperture rate. For this reason, in the liquid crystal display device for a projector requiring a high definition panel in which a pitch of pixels is smaller than 30 μm, the use of metal or the like for avoiding the generation of the transverse electric field becomes a serious problem.
Another means for avoiding the generation of a transverse electric field, is a frame inversion driving method.
This frame inversion driving method is a driving method in which all the polarities of video signals supplied to all pixels within a pixel matrix (hereinafter referred to as pixel signals) are set so as to be identical to one another, and the polarity is inverted every frame.
The description hereinbelow will be given with respect to an example in which a liquid crystal display device using polysilicon TFTs as pixel TFTs is driven by utilizing the frame inversion driving method.
FIG. 1 shows a structure of a liquid crystal display device using polysilicon TFTs as pixel TFTs. This liquid crystal display device is structured so that pixels PEij in which pixel TFTs (a), storage capacities (b) and pixel electrodes (c) are arranged in intersections between longitudinally distributed data lines Dj (n is one of 1, 2, . . . , n) and transversely distributed gate lines Gi (i is one of 1, 2, . . . , m), respectively, to form a matrix. A data driver circuit 112 and a gate driver circuit 114 are arranged in the periphery of the pixel matrix 116. The data driver circuit 112 is the circuit for driving the data lines, and the gate driver circuit 114 is the circuit for driving the gate lines.
The data driver circuit 112 includes switch arrays 119g (g is one of 1, 2, . . . , P, and P is the number of blocks) each serving to individually sample pixel signals supplied through 6 video signal wirings (hereinafter referred to as pixel signal lines) S1 to S6 to corresponding six data lines, respectively, and a scanning circuit 121 for supplying ON/OFF control signals SPg to the switch arrays 119g, respectively. In other words, the data driver circuit 112 is the circuit in which each of the switch arrays 119g is composed of six analog switches, and which serves to carry out the block division driving for simultaneously sampling six pixel signals supplied through the six pixel signal lines S1 to S6, respectively, with the six analog switches as one unit, i.e., as one block.
Timing charts when the above-mentioned liquid crystal display device for a projector is subjected to the frame inversion driving are shown in FIG. 2 and FIG. 3. FIG. 2 is a timing chart in a frame in which pixel signals each having a polarity positive with respect to an electric potential Vcom of a counter electrode of the pixels in the pixel matrix are written, and FIG. 3 is a timing chart in a frame in which pixel signals each having a polarity negative with respect to the electric potential Vcom of the counter electrode of the pixels in the pixel matrix are written.
In FIG. 2 and FIG. 3, DCLK1 and DCLK2 are respectively control clock pulses which are supplied to a shift register (not shown) constituting the scanning circuit 121. The control clock pulse DCLK2 is obtained by inverting the control clock pulse DCLK1. SPg−1, SPg and SPg+1 are respectively ON/OFF control signals which are generated from the shift register in the scanning circuit 121 to which the control clock pulses DCLK1 and DCLK2 are supplied.
The pixel signals supplied through the pixel signal wirings S1 to S6 are respectively sampled by the switch arrays 119g which are turned ON/OFF in accordance with the ON/OFF control signals SPg, respectively, to be outputted to the corresponding six data lines to thereby be used in the display for the pixels.
Japanese published application JP 10-197894 discloses a driving method in which when TFTs for switching are poor in characteristics in a liquid crystal display device for carrying out the block division driving, the number of data lines included in a block is increased to realize the high speed operation.
In addition, a method of manufacturing a polysilicon FET, and a technique for changing a structure to attain a high speed operation for frame inversion driving are described in Japanese published application JP 2001-228457 A.
As described above, the polarities of the pixel signals on the data lines used in display for the pixels are identical to one another within at least one frame time period.
For this reason, if the above-mentioned frame inversion driving is carried out, then a mean value of the pixel signals applied to all the data lines greatly fluctuates depending on the pixel signals. The fluctuation of the mean value causes a difference in the potential fluctuation of the gate lines coupled to the data lines through the parasitic capacities, and the counter electrode. As a result, there is a technical problem in that transverse crosstalk is generated.
In addition, since a mean value of the pixel signals applied to the data lines within one frame (sub-frame) also fluctuates depending on the pixel signals, there is a technical problem in that longitudinal crosstalk is generated.